From the series Beast Wars. Reverting to Beast Mode was when either Predacons or Maximals transformed into their animals forms. In Beast Mode, they were usually able to move faster and transforming also sped up their internal repairs. Also, in the first season, reverting to Beast Mode protected them from Energon poisoning, which was what happened when they were exposed to too much raw energon.

1.A state of or doing anything accessively and or aggressively; wildin' out
2.Over-do-it

"The cops where chasing me down the Ave. so I went into beast mode; jumped over a couple of fences into a few backyards I blew on them and left them 10 blocks behind me."
"After they smoked that ounce of weed, Kelly and Shelly went beast mode on the french fries as a result of having the munchies."
"While we were f***in I went beast mode on her p***y!"